* EVOLUTION 351 



years owes much to the work of W. Bateson (1894), and it 

 is evident that such study is greatly simplified in cases where 

 the variation is discontinuous, that is, where some one or 

 more clear-cut distinctions between '* variety " and " type " 

 can be definitely shown, as in the blackish scaling of the 

 wings of the doubledayaria form of the moth Amphidasys 

 hetularia, or in the white-eyed offspring of normally red- 

 eyed fruit-flies (Drosophila). These are examples of the 

 alternative characters specially suited for the investigation 

 of heredity on the principles of Mendel (pp. 121-131). Now, 

 in the case of some distinctions of this nature there are 

 records of the occurrence of both forms as far back as the 

 species have been studied by naturalists, so that the appear- 

 ance of one or the other is due to the sorting out in the 

 maturation of the germ-cells of factors that have been handed 

 down for a long series of generations. In other cases, how- 

 ever, including those of Amphidasys and Drosophila, just 

 mentioned, the dark scahng of the former and the white 

 eyes of the latter appeared at a definite recorded time in the 

 histor}^ of the race ; in the case of Amphidasys hetularia the 

 new dark-winged variety doubledayaria was observed rather 

 rapidly to replace the light-winged type over considerable 

 areas over northern England. Here, then, is seen a definite 

 example of the appearance and establishment of a new form 

 of insect. During recent years, since attention has been 

 directed by Bateson and others to the importance of dis- 

 continuous variation, and especially since the promulgation 

 of the Mutation Theory of H. de Vries (1901-3) founded on 

 the apparently sudden appearance of new varieties of plants, 

 many students of the subject have attributed a leading part 

 in the evolutionary process to such mutants as are exemplified 

 by the doubledayaria race of Amphidasys betularia and the 

 white-eyed and short-winged forms of Drosophila. The 

 immediate cause of these mutants is to be sought in the 

 origin of some new factor in the germ-plasm, but there is 

 little beyond speculation to suggest how such new factors 

 arise, or even what they are. We know from many carefully 

 conducted breeding experiments that changes in the germ- 



